With over 200,000 new cases yearly, breast cancer is the most common malignancy of women in the United States (US). The past 20 years have seen significant reductions in mortality from breast cancer in the United States and elsewhere. This reduction has been largely due to improvement in early detection and the development of more effective adjuvant therapies. Despite the fact that there have been significant advances in the treatment of breast cancer, the fact remains that once metastatic, the disease remains incurable. Recent studies have provided strong support for the cancer stem cell hypothesis which suggests that breast cancers are driven by a subpopulation of cells which display stem cell properties. These properties include self-renewal which generates other cancer stem cells and differentiation which generates populations of cells forming the bulk of the tumor. There is increasing evidence that cancer stem cells are resistant to chemotherapy and radiation therapy and, thus, contribute to treatment resistance and relapse.
The development of biomarkers to identify CSCs, as well as validation of in vitro and mouse models, has facilitated the isolation and characterization of these cells from both murine and human tumors. Breast cancer stems cells have been characterized by expression of the cell surface markers ESA and CD44 in the absence of expression of the marker CD24. These cells have been termed “breast cancer stem cells” (BCSCs). As few as 200 ESA-positive CD44+/CD24−lin− cells were able to generate tumors in immunocompromised NOD-SCID mice, whereas 100-fold more cells without these markers isolated from the same tumors were non-tumorigenic. Furthermore, the tumor-initiating populations regenerated tumors that recapitulated the heterogeneity of the initial tumor. The art has also developed an in vitro “mammosphere” assay as a means of quantitating normal and malignant stem cells. More recently, the art described the expression of aldehyde dehydrogenase (ALDH) as assessed by the Aldefluor assay (StemCell Technologies, Canada) or the isoform ALDH1 by immunohistochemistry (IHC) as a means of further identifying and enriching for tumor initiating CSC populations in human BCs. Interestingly, it was reported that these markers identify overlapping, but not identical cell populations. Furthermore, it has been found that these markers can be utilized to isolate CSC populations from established breast cancer cell lines, as well as primary tumor xenografts. The development and validation of breast cancer stem cell (BCSC) biomarkers, in vitro mammosphere formation assays, and xenograft models by the art has permitted assessment of chemotherapy and radiation resistance of BCSCs. These studies have demonstrated the relative resistance of BCSC to chemotherapy and radiation therapy. Furthermore, it has recently been demonstrated that the percent of BCSC as assessed either by CD44+/CD24 low mammosphere assays or by ALDH expression increases following neoadjuvant chemotherapy providing direct clinical evidence for the therapeutic resistance of BCSC. Together, these studies suggest that significant improvement in patient outcome will require the successful targeting of BCSCs.